1. Field of the Invention
The present invention relates to a method and system for monitoring cardiac output. In particular, the present invention relates to a method and system for continuously monitoring cardiac output without injection or withdrawal of liquids into the patient.
2. Discussion of Background
Accurately measuring and monitoring cardiac output has long been a clinical and research goal. Both direct and indirect methods are known. Because the history of cardiac monitoring goes back to the early part of this century, there have been many techniques devised. A representative rather than an exhaustive list has been prepared to summarize the methods employed.
One technique known as the "direct method" is the most accurate but is largely restricted to the research laboratory because of the massively invasive or traumatic procedures which must be employed. Less destructive, indirect methods include steady-state Fick oxygen uptake, the transient indicator dilution method, and anemometry.
Of these invasive methods, the transient indicator dilution procedure using iced liquids injected through the lumen of a Swan-Ganz catheter is currently the most frequently employed clinical method. It requires the least amount of specialized equipment, is portable to the patent's bedside and can be repeated often. However, the transient indicator dilution procedure requires a specially trained physician to thread an expensive catheter through the right side of the heart and into the pulmonary artery. During long term monitoring, infection at the site of catheter insertion and damage to the blood vessels of the lung are constant hazards. Swan-Ganz catheters may also need to be repositioned or replaced after a few days of use. Accuracy and repeatability of the thermal dilution Swan-Ganz method are typically no better than 10% even under precisely controlled laboratory conditions.
Non-invasive indirect methods include the ballistocardiography method which requires a patient to lie motionless on a large inertial platform, the soluble gas uptake method which requires a patient to sit in a small chamber for many minutes and the impedance plethysmography method which measures small changes in electrical impedance on the surface of the chest.
The first two non-invasive methods are not readily utilized because the special equipment needed is extremely large and inconvenient to use. In addition, with impedance plethysmography, accuracy is difficult to obtain and thus, it is a less favored method.
Representative heart imaging techniques include 2-D cineangiography and 2D echo cardiography in which a series of x-ray or ultrasound images of the beating heart are measured to determine left ventricle systolic and diastolic volumes. 3-D ECG-gated MRI and radioactive imaging methods where many images of the heart are made during particular phases of the cardiac cycle can also be employed. These methods require large, expensive equipment, and measurements are time consuming and require the efforts of several highly trained specialists to obtain and interpret results.
Therefore, there is a need for a non-invasive and relatively inexpensive device and method for continuously monitoring a patient's cardiac output that can be easily utilized at the bedside or in a physician's office and does not require injection or withdrawal of fluid from the body.